It is known that, in a vehicle engine when the transmission is in the neutral position, warm-up time after startup may be shortened by increasing the idle rotation speed compared to the rotation speed when the vehicle is running.
Tokkai Hei 5-99010 published by the Japanese Patent Office in 1993 discloses a method for varying a target idle rotation speed of the engine provided with a continuously variable transmission. The target idle rotation speed is a control target of idle rotation speed. In this prior art device, the target idle rotation speed changes with an appropriate delay with respect to the gear range of the transmission.
When there is a change-over of gear range between an N range and D range, due to the operating delay of the continuously variable transmission, it takes some time until a new relationship between the engine and propeller shaft is set up. If the target idle rotation speed changes during this time period, the engine rotation speed tends to vary excessively, so to avoid this period, the change-over of target idle rotation speed is delayed relative to when the gear range changes over. In this prior art device, the change-over of gear range was detected as a neutral signal which indicates whether or not the engine is connected to the drive shaft, and the target idle rotation speed was made to vary in synchronism with the delayed signal obtained by performing delay processing on the neutral signal.
Tokkai Hei 5-248301 published by the Japanese Patent Office in 1993 discloses that, when a vehicle with a diesel engine is at rest, the engine exhaust pressure is increased, engine working load is increased and engine cooling water temperature is allowed to rise to improve heating performance of a passenger compartment. For this purpose, a throttle is for example provided in an exhaust pipe, and when a warm up switch operated by the driver is switched ON, this exhaust throttle is closed. After the vehicle starts, the exhaust throttle is opened. To determine whether or not the vehicle is at rest, it is determined whether or not the above-mentioned neutral signal is showing the N range. When the engine is at rest, the engine is usually rotating idle or in a state near to this, and when the exhaust throttle is closed, to prevent the engine rotation from becoming unstable due to rise of exhaust pressure, the fuel supply amount to the engine is increased and the target idle rotation speed is increased.
When this exhaust throttle control is used together with idle rotation speed control according to gear range as disclosed in Tokkai Hei 5-99010, and when exhaust throttle control is performed in the aforesaid delay period, the load change of the engine becomes excessive, and torque shock easily occurs due to the opening and closing of the exhaust throttle.
Also, according to experiments performed by the inventors, it was found that when opening and closing of the exhaust throttle was performed in synchronism with the above-mentioned delay signal for idle rotation speed control depending on the gear range, torque shock was not necessarily reduced.